1. Field of the Invention
The present invention relates to a device and a method for recording optical data to phase-change recording media such as CD-RW, DVD-RAM, DVD-RW, DVD+DW and others.
2. Description of the Related Art
High-speed optical recording media have recently been and continue to be in high demand. Particularly, efforts are being made in increasing the speed of disk-shaped optical recording media, because speed of recording and reproduction of such kind of optical disks can be increased by raising their rotational speed. Among the optical disks, those using optical recording media have simple recording mechanisms because they are capable of recording by only modulating the intensity of the irradiating light, hence it is possible to lower the prices of these media and recording devices. On the other hand, in such kind of devices, since reproduction is achieved also by modulating the intensity of the irradiating light, high compatibility with read-only devices is ensured. As a result of the above facts, optical disks are widely used, and along with the increasing capacity of electronic information, higher density and higher speed optical disks are further required.
Among the optical disks recordable by light-intensity modulation, those using phase-change materials are prevailing because they are rewritable for many times. In an optical disk using a phase-change material, in order to record data, a rapid cooling state and a slow cooling state of the recording material are generated by modulating the intensity of the irradiating light. The recording material is in amorphous state under the rapid cooling condition, and is in a crystalline state under the slow cooling condition. The distinct optical properties of the amorphous state and the crystalline state enable optical information to be recorded.
The principle of recording involves sophisticated mechanisms of rapid cooling and slow cooling of the recording material. As disclosed in the Japanese Unexamined Patent Application Publication (Kokai) No.9-219021, high speed recording is achieved by irradiating a light beam to the recording material with the intensity of the light modulated into three values obtained by pulse division.
When the recording speed becomes higher, however, frequency of the recording channel clock also rises, for example, the frequency of the recording channel clock becomes 104 MHz for 24 times speed CD-RW, and becomes 131 MHz for 5 times speed DVD-RW and DVD+RW. Therefore, in the recording strategy of the related art, the rising times and falling times of the light emitting pulses lengthen, and the effective irradiating energy decreases.
FIGS. 1A through 1C present an example. As shown in FIG. 1A, contrary to the ideal shape of light emitting pulses represented by the dotted line, each actual light emitting pulse requires a rising time and a falling time as indicated by the solid line, and thereby cannot be of a rectangular shape. Further, when frequency of the recording channel clock becomes higher, as shown in FIG. 1B, the rising times and the falling times become longer, and sufficiently high recording power (peak value) and sufficiently low bias power (bottom value) cannot be secured. In other words, the recording power (peak value) Pw decreases by ΔPw, and the bias power (bottom value) Pb increases by ΔPb.
Recording with pulses of such a pulse shape, the recording material cannot be heated and cooled sufficiently, and it is difficult to secure a slow cooling state. Consequently, the recording mark (just abbreviated as “mark” hereinafter) in an amorphous state cannot be formed sufficiently, and the amplitudes of the reproduced signals become low.
This difficulty can be overcome if utilizing a light emitting source of short rising and falling times (a laser diode and a driving device thereof), but it is not easy to guarantee a light emitting source working even at frequencies over 100 MHz.
In the related art, to solve the problem, it is proposed to reduce the number of the recording pulses by methods disclosed in Japanese Unexamined Patent Publication No. 9-134525 and in U.S. Pat. No. 5,732,062.
In detail, FIG. 1B shows the method of recording using three pulses, whereas FIG. 1C shows the recording technique using two pulses. Due to this technique, it is possible to obtain a long light emission time per pulse (duration of the level of Pw) and a long cooling time (duration of the level of Pb), and to reduce the influence of the rising time and the falling time mentioned above.
In this method, however, in order for light emission with a number of m pulses, different optimum light emission patterns have to be set for marks of different lengths. This optimization is difficult, and this method has not been made into practical use.
Japanese Unexamined Patent Publication No. 2001-331936 discloses a method of setting pulse lengths independently and separately. Using this method, high modulations and good jitter characteristics are obtainable even in optical systems having a low recording power and a low responding performance.
As shown above, however, since complicated adjustments are necessary for marks of various lengths, the dependence of the length of the recording mark (just abbreviated as mark length hereinafter) on the recording power is different for different kinds of marks. Consequently, the margin of the recording power is narrow, and the accuracy of the method of setting the optimum recording power (OPC: Optimum Power Control) becomes essential.
In recording methods of the related art, usually the method disclosed in Japanese Unexamined Patent Publication No. 9-138947 is used for setting the recording power, and is thought to be reliable. In the method of reducing the pulse number, the recording power has to be set according to time information associated with mark lengths, because the time information turns out to change more remarkably than the modulation that is a characteristic value of the amplitudes of the reproduced signals.